1. Field of the Invention
The present invention relates to a process for the removal of water from reaction mixtures for the esterification of acids or acid anhydrides with alcohols or for the preparation of alcoholates from aqueous alkali metal hydroxide solutions and alcohols at the boiling point of the reaction mixture, in which the lowest-boiling educt is first employed in less than the stoichiometric amount, the resulting vapor mixture of chiefly water and the lowest-boiling component is freed from water on a membrane, the vapor mixture which has been freed from water is recycled into the reaction mixture and the reaction mixture is topped up with the lowest-boiling educt in the course of the reaction.
2. Description of the Related Art
In chemical equilibrium reactions in which water is formed, it is already known to remove this by pervaporation or vapor permeation with the aid of semipermeable membranes and as a result to shift the equilibrium and thus to bring the reaction to completion. Such membrane processes for fraction of mixtures with organic components are described in detail in the literature (Rautenbach, Chem. Ing. Techn. 61 (1989) pages 539-544). Pervaporation and vapor permeation have also already been described in detail (DE 3 610 011, DE-A 4 019 170 and EP-A 294 827).
Generally, in these processes, the substance mixture to be separated (feed) is fed along a membrane which has different permeabilities to the individual compounds of the approaching substance mixture. The driving force for transportation of matter through the membrane here is a transmembrane difference in the electrochemical potential of the individual substances of the feed mixture. In the case of pervaporation and vapor permeation, this potential difference is accentuated by a vacuum applied to the membrane side facing away from the feed (permeate side) or by flushing the permeate side with inert gas, which has the result of diverting the preferentially permeating components out of the feed. An increased pressure is in general applied to the feed side of the membrane. The depleted feed is called the retained material and the substance amounts passing to the permeate side are called the permeate. The separation properties of membranes are highly temperature-dependent, but are limited by the heat stability of the membrane employed. As, for example, the heat stability of the membrane Pervap 1000 from Deutsche Carbone, GFT, of the polyvinyl alcohol/polyacrylonitrile type is stated as 100.degree. C.
Pervaporation is described, inter alia, by Klatt (dissertation paper "On the use of Pervaporation in the Environment of the Chemical Industry", Rheinisch-Westfalische Technische Hochschule (RWTH) Aachen 1993). Pervaporation is distinguished by the fact that the feed mixture is fed in liquid form just below the boiling point at the given system pressure. The system pressure must be chosen such that, at the membrane, the temperature of the substance mixture to be separated corresponds as far as possible to the optimum temperature in respect of permeate flow and selectivity, but must in no way be above the maximum permitted operating temperature of the membrane (membrane stability). Vapor permeation is described in detail, inter alia, by Helmus (dissertation paper "Vapor permeation-separation capacity, process development and possible uses", RWTH Aachen 1994). In vapor permeation, in contrast to pervaporation, the feed is fed over the membrane in vapor form.
Kitha reports in Chemistry Letters (Chem. Soc. of Japan) 1987 2053-2056 on the esterification of carboxylic acids with alcohols, for example with ethanol, in which ethanol/water corresponding to the thermodynamic equilibrium is evaporated off from the reaction mixture initially introduced, the escaping vapor is fed along a vapor permeation membrane, and the retained material depleted of water is recycled into the reaction mixture. The degrees of conversion which can be achieved are increased up to complete conversion of the carboxylic acid with the process described. The membranes employed are polyimide, chitosan and Nafion membranes, the polyimide membranes employed having adequate selectivities in particular. Gref (Proc. Fourth Int. Conf. Pervaporation, Proc. Chem. Ind. 1989, 344) also describes the use of pervaporation for removing water of reaction from the circulation in the preparation of carboxylic acid esters using PVA membranes. Sander (EP 299 577) describes the use of a PVA membrane for selective removal of water from the vapor of a reaction mixture during the preparation of alcoholates from NaOH and alcohol. Blum (De 4 019 170 A1) reports on the successful use of vapor permeation for removal of water in the preparation of dodecyl acetate and isopropyl myristate (esterification reactions of alcohols with carboxylic acid) in a bubble reactor. EP 0 476 370 describes the use of pervaporation/vapor permeation for removal of water from a mixture comprising water and alcohols and/or carboxylic acids and/or carboxylic acid esters, a membrane which has been obtained by plasma polymerisation being used. EP-A 691 324 describes the use of vapor permeation for the preparation of maleic acid alkyl esters from maleic anhydride and alcohol, the vapor mixture fed to the module comprising condensed contents in order to avoid overheating of the vapor mixture when flowing over the membrane and the associated reduction in the performance of the membrane.
All the procedures described have the common feature that the lowest-boiling educt (in many cases the alcohol) is added at least stoichiometrically, but as a rule in more than the stoichiometric amount. It is attempted by this means to shift the reaction equilibrium to higher degrees of conversion of the acid employed or the alkali metal hydroxide employed, so that the reaction time is simultaneously shortened in this way. Kitha (see above) employs a stoichiometric excess of ethanol of 2:1 and 3:1 in his studies on the esterification of oleic acid with ethanol. Sander (see above) mentions an alcohol excess of 1:2 to 1:10 for the preparation of alcoholates from alkali metal hydroxide and alcohol with removal of the water of reaction from the circulation by vapor permeation (page 2 column 2 line 20). Gref (see above) has systematically investigated the influence of the excess factor in the esterification of propionic acid with isopropanol when pervaporation is employed to remove the water of reaction from the circulation, and comes to the conclusion that a reduction in the excess factor of an educt leads to longer reaction times in respect of the conversion of the minor component (FIG. 13 there). Gref states the stoichiometric use of the educt as the optimum. Blum (see above) also indicates an optimum with stoichiometric use of the educts and use of vapor permeation for removal of the water of reaction from the circulation.